I have seen Serenity, thanks to a kind benefactor who saw my plea on the Browncoats board. I'm cutting my general reactions for the super-paranoid; there are no actual plot or content specifics. Spoilery comments and questions will follow in a separate post.
I need to see it again.
I was distracted, this first time, by the little critic's voice in my head saying things like "check, there's that bit of backstory or character established" or "hmm, that's a slight change from the series, that must be because of X." Also, there's the usual "hey, there's a bit from the trailer!" distraction. Only the latter, obviously, will be a problem for people who haven't seen the TV series.
For those who have seen the series, a great many of the virtues of the show are preserved: tension, humor, pacing (the opening does fabulously at this and I think would get anyone interested, assuming they're in the theater to see it), some lovely character development, and the willingness to put its characters through the grinder. I get very emotionally engaged with on-screen tension, and for large portions I was on the edge of my metaphorical seat (and, on one occasion, literally not looking at part of the screen). There was also the grand fun of catching plot developments half a beat before they were revealed—one of these ran audibly around the theater in a most satisfactory fashion.
This general level of high engagement made even more jarring the jolts into what I think of as "Hollywood moments": the blatant shortcuts to character development, the emotional cliche, the unsubtle speech, the general feeling that a message is being flashed at you in neon. I freely admit that I am more sensitive to such things than a lot of people; also, it was a very long week already by last night. But I fancied I could hear the Hollywood switch being thrown on and off, they seemed to stand out so much to me; and while I am not at my best today [*], it seems to me that there was a noticeably higher proportion of such moments than the TV show had, at least one of which had me saying "who are you and what have you done with X?"
[*] I was amazed and appalled at how much traffic there was in downtown Boston at 12:30 on a Thursday night. Also, I had a migraine earlier today.
I was also considerably distracted, during the rush of the movie itself, by one idiot plot moment and one apparent inconsistency. (I bet 
veejane is saying what, just one of each?)
It's easier to say what one didn't like than what one did. My nitpick threshold appeared to have been set on "high" last night, and a lot of the things that bothered me are either known idiosyncracies or would not bother someone new to the world. I enjoyed the vast majority of the movie very much, and I expect it will be a smoother experience on rewatching. I certainly wouldn't suggest anyone not see it because of my opinion, and I fully expect a lot of people will like it better than I did—and I did like it a lot, honestly, for all my nitpicks.
Notes on the screening itself: the story is final, but the credits, special effects, sound, and music are still being refined. A crew member (second unit director and visual effects supervisor? IMDB isn't throwing up anything that looks right) did a Q&A, which I left partway through because I was tired and my threshold for non-questions was well surpassed. I think the only thing of interest was that there are discussions of TV spinoffs, but not of bringing Firefly itself back to TV.